Conventional DC motor testers include speed-torque-current testers in which data is collected to form a set of constant points on the speed-torque-current operating curves. No time dependent data is collected. This type of tester requires that the motor be coupled to a velocity and a torque measuring system as well as a current measuring system.
The following publication describes a technique known in the prior art as "Pasek's Dynamic Method" of motor testing: W. Lord and J. W. Hwang, "Pasek's Technique for Determining the Parameters of High-Performance DC Motors", Proceedings of the Third Annual Symposium on Incremental Motion Control Systems and Devices, University of Illinois, pp. R.1-10, May 1974.
The "Pasek Dynamic Method" of motor testing is based on the starting current, peak current, peak current time of occurrence, current at twice the time of the peak current occurrence, steady state current and steady state velocity. Pasek's method measures a few specific points on the current time response curve making it very sensitive to current commutation noise, which renders the method inaccurate for low-cost DC motors of the type which would be commonly employed in printers and the like. Pasek's method is in fact only useable on special high quality servo motors. Because of the noise sensitivity problem, it is unknown whether there are any manufacturers who in fact employ a tester based upon this method. In the instance of the present invention, the motor is not coupled to a tachometer or encoder, and the parameters are calculated from more than a few data points and are calculated only from data in which a significant influence is recognized.
Certain of the prior art discusses the utilization of current spikes to measure the speed of a commutated electric motor. For example, U.S. Pat. No. 3,675,126 which issued on July 4, 1972, discloses a current transfer and a band pass filter for sensing power line perturbations caused by motor commutation. The sense perturbations trigger a one shot multivibrator, the on time of which, with respect to total time is taken as a measure of motor speed. Moreover, in U.S. Pat. No. 3,079,523 a speed regulating system for direct current commutating motors is described in which the frequency of commutation is recognized as being proportional to the motor speed. However, none of these patents or any other references disclose the idea of performing a Fourier transform on the steady state current to determine its power- spectral-density, and therefore, the frequency at which maximum power is developed by the commutation.